A. Field of the Invention
The present invention relates generally to the field of wireless communications. More particularly, the present invention relates to LO leakage and sideband image calibration for a wireless communication system, and more specifically for an RF transmitter.
B. Background
Recently, the demand for wireless communication systems has grown significantly, such as for wireless local area networks (LAN), home wireless control systems and wireless multimedia centers. Along with this growth in demand, there has also been a concomitant increased interest in more bandwidth, more powerful and cheaper chip systems. For example, the maximum 11 Mb/s bandwidth offered by the 802.11b standard cannot satisfy the requirements of most users. Rather, a higher bandwidth chip, such as a 54 Mb/s chip offered by the 802.11g or 11a standard, is preferred. To transmit in this bandwidth with just a 20 MHz wide channel, more advanced modulation methods need to be adopted. In a 802.11a/g standard wide area network LAN (WLAN) system, the data is modulated with binary phase-shift-keying (BPSK), or quadrature phase-shift-keying (QPSK), or 16-level or 64-level quadrature amplitude modulation (16-ary QAM or 64-ary QAM), and further mapped into an orthogonal-frequency-division-multiplexing (OFDM) signal with 52 sub-carriers.
To take the advantage of the high bandwidth, an OFDM-based wireless system and a radio wave system with sophisticated modulation techniques pose significant implementation challenges requiring low in-band phase noise, high linearity and accurate matching of the RF transceiver chip. Among them, matching is the most complicated issue because it arises from device mismatch. Furthermore, the requirement for the chip mismatch is typically very tight as well. For example, in order to meet the transmitter Error Vector Magnitude (EVM) specification for 54 Mb/s mode in a WLAN system with a 3-dB implementation margin, which is the parameter to indicate the quality of a digital modulated signal, system simulation shows that an I/Q mismatch less than 1.5°/0.2 dB is required. Also, the transmitter LO leakage introduced by mismatch is imposed as unwanted signals, and therefore should be as small as possible in order to lessen any interference and noise problems.
An RF transmitter performs baseband signal modulation, up-conversion and power amplification. Compared with various approaches to implement an RF receiver, only a few architectures are currently available for a transmitter realization. This is because the noise, interferences rejection and band selection are more relaxed for a transmitter than they are for a receiver in a wireless communication system.
If the transmitted carrier frequency is equal to the local oscillator (LO) frequency, such an architecture is termed a “direct conversion” architecture. As shown in FIG. 1, which corresponds to a direct conversion transmitter architecture, the signal modulation and up-conversion occur in a same circuit, whereby that same circuit corresponds to a mixer 110 and a local oscillator 120. The mixer 110 receives an LO signal from the LO 120, and mixes the LO signal with a baseband signal output by a baseband filter 105. The output of the mixer 110 is provided to a power amplifier 140, which provides sufficient power that is transferred to an antenna (not shown) and filters out-of-band components that result from nonlinearities.
However, some undesired in-band signals are introduced by circuit defects, such as quadrature LO signal amplitude and phase mismatch, baseband signal amplitude and phase mismatch, and device mismatch. Among them, the LO leakage and sideband modulation image mismatch are the most critical ones to degrade the transmission signal quality, whereby they respectively correspond to the leakage power at the exact LO frequency (fLO) and at the sideband introduced by the quadrature mismatch. Both of them exist in the transmission band, and thus the power amplifier 140 and any following band pass filter (not shown) cannot filter them out.
Another approach to up-convert a baseband signal to an RF frequency is to modulate the signal in two or more steps so that the output spectrum is far from the local oscillation frequency, which results in immunity of the frequency synthesizer to frequency pulling. FIG. 2 shows a two-step conversion transmitter architecture, whereby the baseband signal BBI undergoes quadrature modulation at a low frequency called an intermediate frequency (IF), and whereby that result is upconverted to the desired frequency through bandpass filtering and mixing with another LO frequency, which suppresses the harmonics of the IF signal. In more detail, a first local oscillator 220 provides a first LO signal LO1, which is mixed with the filtered baseband signal (a filter 205 filters the baseband signal BBI) by way of a first mixer 230. The output of the first mixer 230, which is at an IF band, is provided to an off-chip filter 240, whereby this completes the first step. The output of the off-chip filter 240 is provided to a second mixer 250, whereby it is mixed with a second LO signal LO2 output by a second local oscillator 255, in order to provide an RF signal. The RF signal is amplified by a power amplifier 260, to thereby provide an output RF signal RFO.
The advantage of this two-step upconversion over the direct conversion approach is that the signal quadrature mismatch is better because the modulation is performed at a low frequency (IF) as opposed to a high frequency (RF). However, the rejection of the unwanted sideband generated by the quadrature upconversion due to the mismatch is very tight, typically 50 to 60 dB. Also, the LO leakage is required to be very small because of Federal Communication Commission (FCC) requirements and other requirements.
No matter which transmitter architecture is used, the small mismatch of circuits and signals is preferred, to thereby allow more complicated modulation techniques to be used and which provides a higher transmission efficiency. However, device mismatch always exists and cannot be completely removed. Thus, precise calibration is required to improve the transmitter performance.
Several calibration methods exist for RF transmitters in order to perform amplitude and phase calibration in a wireless system, whereby they can be divided into two categories in terms of the detection and calibration locations. In a first category, both the mismatch detection and calibration are done by a digital baseband. As shown in FIG. 3, a digital baseband circuit 310 transmits a pilot sequence to an RF transceiver 320, and is modulated into a high frequency by the RF transceiver 320. A D/A 340 and an A/D 350 provide digital/analog signal conversion between these two components. During calibration, the RF transceiver 320 connects the transmitter output to the receiver input so that the digital baseband circuit 310 can receive the demodulated signal at the same time. A digital signal processing (DSP) machine (not shown) inside the digital baseband circuit 310 is required to calculate the phase and amplitude mismatch of the RF link and generate the “error” signals to calibrate it into a “perfect” (non-mismatched) communication channel. Although the calibration is relatively easy to control in the digital domain, it imposes several disadvantages: 1) the DSP machine can take a long computation time to get the desired accuracy; 2) it cannot work without the receiver (a part of the RF transceiver 320); 3) the mismatch introduced by the receiver has to be considered, which is comparable with the mismatch from the transmitter (a part of the RF transceiver 320); 4) the hardware mismatch is intact and the performance degradation introduced by the mismatch, such as second order inter-modulation, still exists; and 5) extra connection between the transmitter and the receiver inside the RF transceiver 320 complicates the design.
In the second category of conventional calibration .methods, a digital baseband circuit is used to detect the signal mismatch while a specific circuit inside an RF transceiver performs the calibration under control of the digital baseband circuit. As shown in FIG. 4, the mismatch is detected by a digital baseband circuit 410 similar to previous case shown in FIG. 3, while the mismatch is calibrated out inside an RF transceiver 420 that is controlled by the digital baseband circuit 410. The disadvantages of this type of calibration are: 1) large analog-to-digital converter (ADC) needed with a DSP machine (internal to the digital baseband circuit 410) and long computation time required; 2) dependence on an RF receiver (a part of the RF transceiver 420); 3) introduction of the receiver mismatch; and 4) required extra connection between the transmitter and the receiver inside the RF transceiver 420 complicates the design.